The present invention relates to optical fibers and, more particularly, to providing very accurate dispersion compensation over an entire range of wavelengths.
Dispersion in a glass fiber causes pulse spreading for pulses that include a range of wavelengths, due to the fact that the speed of light in a glass fiber is a function of the transmission wavelength of the light. Pulse broadening is a function of the fiber dispersion, the fiber length and the spectral width of the light source. Dispersion for individual fibers is generally illustrated using a graph having dispersion on the vertical axis (in units of picoseconds (ps) per nanometer (nm), or ps/nm) or ps/nm-km (kilometer) and wavelength on the horizontal axis. There can be both positive and negative dispersion, so the vertical axis may range from, for example, xe2x88x92250 to +250 ps. The wavelength on the horizontal axis at which the dispersion equals zero corresponds to the highest bandwidth for the fiber. However, this wavelength typically does not coincide with the wavelength at which the fiber transmits light with minimum attenuation.
For example, typical single mode fibers generally transmit best (i.e., with minimum attenuation) at 1550 nm, whereas dispersion for the same fiber would be approximately zero at 1310 nm. The theoretical minimum loss for glass fiber is approximately 0.16 db/km, and that occurs at the transmission wavelength of about 1550 nm. Because minimum attenuation is prioritized over zero dispersion, the wavelength normally used to transmit over such fibers is typically 1550 nm. Also, Erbium-doped amplifiers, which currently are the most commonly used optical amplifiers for amplifying optical signals carried on a fiber, operate in 1530 to 1565 nm range. Because dispersion for such a fiber normally will not be zero at a transmission wavelength of 1550 nm, attempts are constantly being made to improve dispersion compensation over the transmission path in order to provide best overall system performance (i.e., low optical loss and low dispersion).
Many techniques have been used for dispersion compensation, including the design and use of dispersion-shifted and dispersion flattened fibers. Dispersion Compensating Modules (DCMs) have also been used in optical communications systems for dispersion compensation, especially in wavelength division multiplexing (WDM) systems. A number of patents describe various uses of DCMs to compensate dispersion including: U.S. Pat. No. 4,261,639 (Kogelnik et al.); U.S. Pat. No. 4,969,710 (Tick et al.); U.S. Pat. No. 5,191,631 (Rosenberg); and U.S. Pat. No. 5,430,822 (Shigematsu et al.). These patents compensate dispersion by inserting DCMs at appropriate intervals along the transmission path. The DCMs usually contain Dispersion Compensating Fiber (DCF) of an appropriate length to produce dispersion of approximate equal magnitude (but opposite sign) to that of the transmission fiber.
One problem with using the known DCMs to compensate dispersion is that DCF designs are very sensitive to production tolerances. Therefore, if the DCF design is not highly precise, then when the DCF is combined with the transmission fiber, the resulting transmission link may have too much residual dispersion (i.e., dispersion on wavelength channels other than the center wavelength channel being compensated). This is especially true in broadband applications where the transmission rates may be, for example, 40 gigabits per second (Gbit/s). Also, once the DCF is produced, only the length of the DCF can be selected to meet the desired target for dispersion compensation. Moreover, selection of the DCF length (and thus the dispersion of the DCM) should ensure that first order and higher order dispersion are compensated.
When compensating for higher order dispersion, it is very important that the Relative Dispersion Slope (RDS) of the transmission fiber match the RDS of the DCF (and consequently of the corresponding DCM). For a given fiber, the RDS is defined as the ratio of the dispersion slope, S, of the fiber to the dispersion, D, of the fiber. Thus, the RDS for a given fiber is equal to S/D for that fiber. For a DCF combined with a transmission fiber, the total dispersion and the total dispersion slope of the compensated link, DLINK and SLINK, respectively, can be expressed by Equations 1 and 2, respectively, as follows:
DLink=DTransmFiberxc3x97LTransmFiber+DDCFxc3x97LDCFxe2x80x83xe2x80x83(Equation 1)
SLink=STransmFiberxc3x97LTransmFiber+SDCFxc3x97LDCFxe2x80x83xe2x80x83(Equation 2)
In Equation 1, DTransmFiber corresponds to the dispersion of the transmission fiber, LDCF corresponds to the length of the DCF, and DDCF corresponds to the dispersion of the DCF. In Equations 1 and 2, LTransmFiber corresponds to the length of the transmission fiber and LDCF corresponds to the length of the DCF. In Equation 2, STransmFiber corresponds to the dispersion slope of the transmission fiber and SDCF corresponds to the dispersion slope of the DCF.
When the dispersion of the system is compensated, i.e., when DLink=0 (i.e., when DLINK is set equal to 0 for purposes of calculations), the length of DCF needed to compensate for the dispersion slope and the dispersion of the link can be determined by Equation 3. Because the values of the DCF dispersion, the transmission fiber dispersion, and the transmission fiber link are known, the length of DCF needed is given by:
LDCF=xe2x88x92(DTransmFiber/DDCF)xc3x97LTransmFiber.xe2x80x83xe2x80x83(Equation 3)
In order to compensate the link for the dispersion slope, SDCF, of the DCF itself, the RDS for the DCF and for the transmission fiber must be matched such that:                               RDS                      Trams            .            Fiber                          =                                            S                              Trams                .                Fiber                                                    D                              Trams                .                Fiber                                              =                                                    S                DCF                                            D                DCF                                      =                          RDS              DCF                                                          (                  Equation          ⁢                      xe2x80x83                    ⁢          4                )            
However, when producing a DCF, the production tolerances inherently cause variations in the dispersion and in the RDS of the DCF. Dispersion variations at the center wavelength can be compensated by choosing the correct length of DCF, but RDS variations are not compensated. Tolerances on RDS for DCF are typically about xc2x115%, which can cause significant residual dispersion at the edges of the transmission band, which is undesirable for the aforementioned reasons.
A technique that uses DCM technology for improving dispersion compensation over a center wavelength and for reducing residual dispersion on wavelengths at the edges of the transmission band is disclosed in U.S. Pat. No. 5,781,673 (hereinafter the ""673 patent), which is assigned to the assignee of the present invention, and which is incorporated herein by reference in its entirety. This patent discloses a wavelength division multiplexing (WDM) system in which the transmission path between a WDM receiver and a WDM transmitter comprises a transmission fiber of a particular length having a particular dispersion of a particular sign combined with a DCF of a particular length and having a particular dispersion of opposite sign as that of the dispersion of the transmission fiber. This combination ensures that the center wavelength of the channel will have a nominally zero overall dispersion.
In order to compensate for the residual dispersion on the other channels, the ""673 patent discloses adding to the link a dispersion slope compensating fiber (DSCF) of a particular length and having a relatively large negative dispersion slope and a nominally zero dispersion. The dispersion slope of the DSCF is calculated as the sum of the residual dispersions on the extreme channels of the transmission path divided by the wavelength difference between the extreme channels.
Although the technique disclosed in the ""673 patent improves dispersion compensation in broadband applications, a need to further improve dispersion compensation in various applications, such as broadband applications, still exists.
The present invention provides a dispersion compensation module (DCM) for compensating dispersion of an optical fiber transmission link is provided. The optical fiber transmission link comprises a transmission fiber and the DCM. The DCM comprises at least first and second dispersion compensating fibers, DCF1 and DCF2, respectively. DCF1 and DCF2 each have a dispersion, D1 and D2, respectively, a dispersion slope, S1 and S2, respectively, and a relative dispersion slope, RDS1 and RDS2, respectively. The transmission fiber also has a dispersion, DTransFiber, a dispersion slope, STransFiber, and a relative dispersion slope, RDSTransFiber. DCF1 and DCF2 are selected based on their respective relative dispersion slopes, RDS1 and RDS2, respectively. DCF1 and DCF2 have particular lengths, L1 and L2, respectively. The DCFs are combined with each other and with the transmission fiber and the combination results in overall dispersion compensation of the optical fiber transmission link.
The present invention also comprises a transmission system comprising at least first and second dispersion compensation fibers, DCF1 and DCF2, respectively, which are combined in the DCM, and with a transmission fiber. DCF1 and DCF2 are selected based on their respective relative dispersion slopes, RDS1 and RDS2, respectively, such that when DCF1 and DCF2 are combined, the DCM is provided with an effective relative dispersion slope, RDS_DCM. has an effective RDS that matches the magnitude of the RDS of the transmission fiber, but is of opposite side. When the lengths of each of the DCFs are combined with each other and with the transmission fiber, RDS1 and RDS2 are such that the combination of the transmission fiber with the combined DCFs results in overall dispersion compensation of the optical fiber transmission link of the transmission system.
The present invention also provides a method for performing dispersion compensation. The method comprises the steps of selecting at least first and second dispersion compensating fibers, DCF1 and DCF2, respectively, each having a dispersion, D1 and D2, respectively, a dispersion slope, S1 and S2, respectively, and a relative dispersion slope, RDS1 and RDS2, respectively. Once DCF1 and DCF2 and their lengths have been selected, the DCFs are combined with each other and with a transmission fiber. The transmission fiber also has a dispersion, DTransFiber, a dispersion slope, STransFiber, and a relative dispersion slope, RDSTransFiber. DCF1 and DCF2 are selected based on their respective relative dispersion slopes, RDS1 and RDS2, such that the combination of the transmission fiber with the combined DCFs results in overall dispersion compensation of the optical fiber transmission link.
These and other features and advantages of the present invention will become apparent from the following description, drawings and claims.